1. Technical Field
Embodiments described herein relate to a thermal treatment method of a silicon wafer and a silicon wafer.
2. Related Art
For a silicon wafer that is used as a substrate for forming a semiconductor device, there is a demand for an effort to reduce void defects, such as crystal originated particles (COP) or laser scattering tomography defects (LSTD), in the vicinity of the surface of a wafer (hereinafter referred to as the surface layer portion) which acts as a device active area so as to produce a defect-free wafer.
In recent years, a technique has been known as a method of manufacturing the above silicon wafer with a high productivity in which a rapid thermal process (hereinafter also referred to simply as the RTP) is carried out on a silicon wafer having at least the surface on which a semiconductor device is formed mirror-polished (hereinafter the mirror-polished surface will also be termed a polished surface).
As such a technique, JP-T-2001-509319 discloses a thermal treatment method in which a wafer is heated in an atmosphere of an oxygen-containing gas, which is mainly argon or helium, (the inert gas atmosphere termed in the present invention) at a temperature exceeding approximately 1175° C. and an oxygen partial pressure of less than approximately 5000 ppm for less than 60 seconds.
However, in the method as described in JP-T-2001-509319, since the RTP is carried out in an atmosphere of inert gas, such as argon or helium, void defects in the surface layer portion of a wafer can be significantly reduced; however, in a case in which the RTP is carried out at a high temperature exceeding 1175° C. in the above inert gas atmosphere or an atmosphere of reducing gas, such as hydrogen, there is a problem in that the surface roughness of the polished surface is worsened due to the influence of a natural oxidation film foil led on the polished surface of the wafer before the RTP.
Regarding the above problem, JP-A-2000-91342 discloses a thermal treatment method in which the natural oxidation film on the wafer surface is removed by a hydrofluoric acid treatment, and then a thermal treatment is carried out using an RTP apparatus in a 100% hydrogen atmosphere or a mixed gas atmosphere of argon containing 10% or more of hydrogen, whereby the micro-roughness of the wafer surface is decreased, and void defects present on the wafer surface can be removed.
In the method as described in JP-A-2000-91342, since silicon atoms on the wafer surface are terminated with hydrogen by the hydrofluoric acid treatment, it becomes difficult to form the natural oxidation film on the surface. Therefore, deterioration of the surface roughness on the wafer surface can be suppressed even when the RTP is carried out.
However, in order to clear the void defects present on the surface layer portion of the wafer by the RTP, it is necessary to carry out a high-temperature thermal treatment at a minimum of 1000° C. or higher in the inert gas atmosphere or a reducing gas atmosphere, but the bond of the hydrogen atoms terminating the silicon atoms becomes liable to be broken, and the silicon atoms become liable to be exposed on the wafer surface. The silicon atoms exposed in the above manner are unstable and liable to bond with other atoms.
Therefore, for example, when other reactive gas (nitrogen or the like) is present in the above atmosphere, since the reactive gas reacts and bonds with the exposed silicon atoms, and a phenomenon in which the bonds are etched by the atmosphere repeatedly occurs, there are problems in that the surface shape of the wafer is deformed, and the surface roughness is deteriorated.
Furthermore, in a case in which a small amount of oxygen is included in the atmosphere, the exposed silicon atoms react with the oxygen, and an oxidation film is formed on the wafer surface in an island shape. The oxidation film is etched by the atmosphere, but there is another problem in that recess-shaped pits are formed at the etched portions.
The above problems become more remarkable as the thermal treatment temperature is increased in the RTP; however, on the other hand, an increase in the thermal treatment temperature has an advantage of an enhancement of the clearing force of the void defects on the surface layer portion of the wafer.
In addition, the above problems show the same tendency even in a semiconductor device-forming thermal treatment for forming a semiconductor device on the surface of a silicon wafer.
That is, in a case in which the thermal treatment is carried out at a high temperature (for example, 1000° C. or higher) in the semiconductor device-forming thermal treatment, similarly, there is a problem of deterioration of the surface roughness, formation of recess-shaped pits, or the like.